I had doubts first in buying this lens as it was way above my budget but i needed a walkabout lens as i recently sold off my d3000 with the lens and upgraded to a D90. Been using a 18-135 which if compared to this lens, it would be comparing Mc Donald's to a Michelin 3 star restaurant.
Well it was Mr Keane who pushed me off the fence to in the end get this lens, was considering an 18-70 as it was the fastest focusing lens in it's class. But I am glad to say now i have no regrets in spending twice the amount of money for this lens. Bought it second hand and the seller provided me with a free Hoya Pro1-D filter. How does this compare to the Nikon 17-55 f2.8? Well all i can say is : Almost incomparable.
When shot wide open at f/2.8 it gives an ultra sharp image with a really creamy bokeh. For the price of 3x cheaper than the Nikon Original i would say this lens is a steal.
Like any Tamron lens (I have a 70-300mm and I compared it's tone quality and colours to the 17-50), it gives very vibrant and punchy colours. Of course the sharpness of the images by this lens is in a class of it's own. The auto-focus is really fast and accurate at about 98% of the time. The images do not have any chromatic abbreviation or vignette and is stable at all focal lengths. I bought the non-VC (Tamron's equivalent to Nikon's VR) version and to be honest i have not encountered serious handshake in images taken at 1/80 sec (With VC it allows 2-3 stops lower) well anything lower than that, just use tripod hor.
The only thing annoying about this lens is that it's motor tends to be a little noisy at times, but hey, image wise : almost incomparable to the Nikon 17-55. This image is shot wide open at f2.8 and as usual, the bokeh is really something to look at.
Taken at f/4 due to the serious bokehness, produces ultra sharp images eventhough cropped at 80%. A really good lens and I would recommend it to camera users who are looking to upgrade their lens. Just did a shoot assignment for Mr Keane's better half parent's kindergarden's sports day and i have like 645 pics to clear will do a separate post here as all of the pics were shot with the 17-50 so the next post would be a very good review on the lens.